1. Field of the Invention
The present invention relates to a display device that drives a plurality of light-emitting elements individually through driving circuits provided one for each light-emitting element, and to an inspection circuit suitable for the inspection of such driving circuits.
2. Description of the Prior Art
Display devices are known that are provided with a plurality of light-emitting elements arranged in a predetermined pattern or in a regular matrix (array) and that for desired displayed images by controlling the turning on and off of the light-emitting elements individually.
A typical example of a display device hating light-emitting elements arranged in a predetermined pattern is a 7-segment light-emitting diode (LED) display device that displays numerals and alphabets with seven picture elements. With such a display device, which has a relatively small number picture elements, it is customary to control the turning on and off of the light-emitting elements individually through driving circuits provided one for each light-emitting element.
Another example is a fluorescent display tube, which is a display device that has not only light-emitting elements arranged in a matrix but also fluorescent spots serving as picture elements arranged in rows and columns and that makes them emit light by the action of thermoelectrons. Conventionally, in a fluorescent display tube, picture elements that are to be turned on are selected through time-division driving achieved by the use of strip-shaped electrodes. In recent years, however, to avoid inconveniences inherent in time-division driving, it is common to provide driving circuits one for each picture element to control the turning on and off of the light-emitting elements individually.
By providing driving circuits one for each picture element, it is possible to reduce the driving voltage, which leads to many advantages such as reduction in the power consumption, reduction in the heat generated, simplification of the driving circuits, and an extended working life of the fluorescent spots. On the other hand, however, exactly because this construction requires as many driving circuits as picture elements, it inevitably requires too many driving circuits to display high-resolution images.
A fluorescent display tube that has driving circuits one for each picture element is constructed as follows. On a semiconductor chip, driving circuits and electrodes are formed in a matrix, and, over the surfaces of these electrodes, fluorescent material is applied. A hot cathode is placed opposite this semiconductor chip, with a grid between them, and all of these components are housed in a tube, which is then evacuated of air. The electrodes are formed on the surfaces of the semiconductor layers at the top of the driving circuits, and the voltages at the electrodes are varied by the driving circuits. Such changes in the voltages at the electrodes cause changes in the number of thermoelectrons striking the fluorescent spots, and thereby the emission of light is controlled.
Usually, driving circuits formed on a semiconductor chip are inspected by touching the output pad of each driving circuit with a probe to measure the voltage there. However, in the fluorescent display tube described above, it is extremely difficult to use this inspection method, because the driving circuits are connected directly to the electrodes without using pads or the like, that is, the electrodes are formed immediately on the top of the driving circuits, and therefore touching the electrodes with a probe results in damaging the driving circuits.
For this reason, in a fluorescent display tube of this type, the driving circuits are inspected by checking actual emission of light, that is, by feeding driving voltages to the electrodes to actually drive the driving circuits and checking whether each fluorescent spot emits light or not through observation with the unaided eye or through measurement with an optical instrument.
However, in a fluorescent display tube, emission of light is possible only after the application of fluorescent material to the electrodes, the arrangement of the hot cathode, and the evacuation of air from the tube. In other words, the fluorescent display tube can emit light only after it has been finished as an end product. This means that it is practically impossible to inspect the driving circuits in the middle of manufacture. As a result, it is inevitable to finish a product first of all by going through all manufacturing steps even if there are defects in the driving circuits, and thus it is impossible to improve productivity.
In addition, it is not easy to check emission of light with the unaided eye. In a display tube with a relatively small number of picture elements over a relatively wide area, it is possible to observe emission of light from all picture elements at a time. However, in a display tube with many picture elements arranged close to one another, when all picture elements are turned on at a time, some defective picture elements may go unnoticed even if they do not emit light. Accordingly, it is necessary to make the picture elements emit light individually, and thus inspection requires much labor and time. Using an optical instrument for light detection improves inspection efficiency, but, in a fluorescent display tube with many picture elements arranged close to one another, it is all the same necessary to make the picture elements emit light individually, and thus inspection requires much time.
On the other hand, with the 7-segment LED display device mentioned earlier, where the driving circuits are provided with output pads, it is possible to adopt the inspection method that uses a probe. Accordingly, it is possible to inspect the driving circuits without actually effecting light emission. However, it is necessary to measure the voltage at the output of each driving circuit by touching one output pad after another with the probe, and therefore inspection requires more time as there are more picture elements.
As described above, in a conventional display device that has driving circuits one for each light-emitting element, inspection of the driving circuits is not easy, and, depending on the light emission method that the display device adopts, inspection of the driving circuit is simply impossible until the completion of manufacture. In addition, in a display device that has a large number of light-emitting elements, inspection requires too much time.